1. Field of the Invention
The invention relates generally to fluid characterization using nuclear magnetic resonance (NMR) instruments.
2. Background Art
The oil and gas industry has developed various tools capable of determining and predicting earth formation properties. Among different types of tools, nuclear magnetic resonance (NMR) instruments have proven to be invaluable. NMR instruments can be used to determine formation properties, such as the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space. A general background of NMR well logging is described in U.S. Pat. No. 6,140,817.
Nuclear magnetic resonance is a phenomenon occurring in a selected group of nuclei having magnetic nuclear moments, i.e., non-zero spin quantum numbers. When these nuclei are placed in a magnetic field (Bo, “Zeeman field”), they each precess around the axis of the Bo field with a specific frequency, the Larmor frequency (ωo), which is a characteristic property of each nuclear species and depends on the gyromagnetic ratio (γ) of the nucleus and the magnetic field strength (Bo) effective at the location of the nucleus, i.e., ωo=γBo.
Borehole fluid sampling and testing tools such as Schlumberger's Modular Dynamics Testing (MDT) Tool can provide important information on the type and properties of reservoir fluids in addition to providing measurements of reservoir pressure. These tools may perform measurements of the fluid properties downhole, using sensor modules on board the tools. Alternatively, these tools can withdraw fluid samples from the reservoir that can be collected in bottles and brought to the surface for analysis. The collected samples are routinely sent to fluid properties laboratories for analysis of physical properties that include, among other things, oil viscosity, gas-oil ratio, mass density or API gravity, molecular composition, H2S, asphaltenes, resins, and various other impurity concentrations. However, the laboratory data may not be useful or relevant to the reservoir fluid properties because the samples may be contaminated by mud filtrate.
For example, the collected fluid samples could be emulsions of filtrate water and crude oil or, in wells drilled with oil-base muds, mixtures of reservoir crude oil and oil-base mud filtrate. In either case the contamination may render the measured laboratory data irrelevant to the actual properties of the in situ reservoir fluids. In those cases where the samples brought to the surface have low or negligible contamination, laboratory results can still be tainted (e.g., by precipitation of solids caused by temperature changes). Therefore, it is desirable that formation fluid analysis be performed, on uncontaminated samples, under downhole conditions.
Several U.S. Patents disclose methods and apparatus for making NMR measurements in a borehole on fluid samples withdrawn from earth formations. U.S. Pat. No. 6,346,813 B1 issued to Kleinberg and published U.S. Patent Application No. 2003/0006768 by Kleinberg et al. disclose an NMR module on the flowline of the MDT tool for determining different fluid properties from magnetic resonance signals. The Kleinberg patent and the Kleinberg application are assigned to the assignee of the present invention. This patent and the application are hereby incorporated by reference in their entirety. U.S. Pat. No. 6,111,408 issued to Blades et al. also discloses an NMR module that can be incorporated into a fluid sampling tool similar to the Schlumberger MDT™ tool. The NMR module can perform different NMR measurements including T1 (longitudinal relaxation time), T2 (transverse relaxation time), and D (diffusion constant). U.S. Pat. No. 6,111,409 issued to Edwards et al. discloses an apparatus and method for measuring spin-echo signals from carbon-13 nuclei and a spectral method of analyzing these measurements to determine the ratio of aliphatic to aromatic hydrocarbons from the chemical shift spectrum. U.S. Pat. No. 3,528,000 issued to Schwede discloses different methods, including those using an open hole packer, for withdrawing fluid samples into a borehole tool in order to perform NMR measurements on the fluid samples. However, none of these prior art patents disclose methods for determining flow speed or T1 of flowing fluids.
U.S. Pat. No. 6,046,587 issued to King and Santos discloses methods and apparatus for measuring flow speed of multiphase fluids flowing in a pipeline. The King and Santos patent teaches using the ratio of NMR free-induction decay (FID) amplitudes of signals acquired with different delay times to infer flow speed using a single NMR sensor. However, the FID signal is difficult to measure using permanent magnets because static magnetic field variations will result in signals that decay too fast for reliable detection. Moreover, the King and Santos methods for determining flow speed do not account for the fact that there is a distribution of flow speeds in a pipe. In another embodiment, the King and Santos apparatus consists of two separated NMR sensors. The flow speed and fluid volumes for multiphase fluid flow are computed from FID measurements in the two sensors. However, the computation requires prior knowledge of the fluid T1 distributions.
Two methods for measuring the speed of fluids flowing through a flowline of a fluid sampling tool are described in a U.S. patent application Ser. No. 10/349011, filed by Madio et al. on Jan. 22, 2003. This application is assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety. Both methods require that the static magnetic field of the NMR apparatus have a gradient in the direction of the flow. The first method exploits the fact there is a Larmor frequency shift between different wait time measurements in a variable wait time pulse sequence. The measured frequency shifts are proportional to both the flow speed and the gradient. The second method is based on the fact that the measured phase difference between odd and even echoes is proportional to both the average flow speed and the gradient.
While these prior art methods are useful for determining formation fluid properties, there remains a need for better methods for measuring a property of a fluid flowing in a formation testing tool using NMR instruments.